To address the national strategic demand for self-reliant industrial software, a unified-mesh-based one-way thermomechanical coupling algorithm was proposed, and dedicated finite element simulation software for casting temperature-stress analysis was independently developed, providing a reliable numerical tool for process optimization. Through air-cooling simulations of a stepped test block, comparative results with ANSYS demonstrate a maximum relative error of 0.789% with consistent cooling trends, validating the high accuracy of the temperature module. Numerical simulation of a grid-shaped stress framework reveals deformation behavior, stress-strain distributions, and hot tearing tendencies during solidification, confirming the correctness of the thermomechanical coupling algorithm and mechanical boundary model. For a combustion chamber shell casting case, the predicted hot tearing defects match actual cast products, confirming the software's industrial applicability for optimizing practical large-scale casting processes.